1. Field of the Invention
The invention relates a cold cathode fluorescent lamp driving apparatus using a piezoelectric transformer which transforms the amplitude of an AC voltage by the piezoelectric effect of a piezoelectric element such as a piezoelectric ceramics.
2. Related Art of the Invention
A piezoelectric transformer developed at the end of the nineteen fifties was further developed because it received attention as a step-up transformer for a high voltage power source. However, material restrictions such as a breaking strength of a piezoelectric ceramic material prevented a piezoelectric transformer from being greatly commercially introduced, and its development was suspended. In recent years, development of a high-strength piezoelectric ceramic progresses, and portable information devices such as note-type personal computers, electronic organizers, and game machines are significantly required to be smaller and thinner. With such development and requirements, great attention is again directed toward a piezoelectric transformer as a step-up transformer in an inverter power source for a liquid crystal back light which is mounted on such a device.
An inverter for a back light is used as a power source for a cold cathode fluorescent lamp which is used as a source for a back light. The inverter requires transformation of a low DC voltage such as 5 V, 9 V, or 12 V supplied from a battery to a high-frequency voltage of a high voltage of about 1,000 Vrms at a start of the lighting and of about 500 Vrms in a steady state. An electromagnetic wound-type transformer which is currently used in an inverter for a back light utilizes a horizontal structure having a special core so as to comply with a tendency to a thinner body. In order to ensure a withstand voltage, however, there is a limit for realizing a smaller and thinner transformer. In addition, because the core loss is large and the use of a thin copper wire causes the winding loss to be increased, the efficiency is disadvantageously low.
On the other hand, a piezoelectric transformer has the following configuration. Primary (input side) and secondary (output side) electrodes are disposed on a piezoelectric ceramic material such as lead zirconate titanate (PZT) or a piezoelectric crystal material such as lithium niobate. An AC voltage of a frequency which is in the vicinity of the resonance frequency of the piezoelectric transformer is applied to the primary side so that the piezoelectric transformer is caused to mechanically resonate. The mechanical oscillation is transformed by the piezoelectric effect so as to be taken out from the electrode on the secondary side in the form of a high-voltage power. Such a piezoelectric transformer can realize a smaller body, and especially a thinner body as compared with an electromagnetic transformer. In addition, the piezoelectric transformer can attain a high conversion efficiency.
Hereinafter, a prior art cold cathode fluorescent lamp driving apparatus using a piezoelectric transformer will be described with reference to the relevant drawings.
FIG. 20 is a view schematically showing a Rosen-type piezoelectric transformer. The piezoelectric transformer is constructed in such a manner that electrodes on the primary side (input side) and the secondary side (output side) are disposed on a rectangular plate made of a piezoelectric ceramic material such as lead zirconate titanate (PZT). As indicated by P in the figure, the primary side is polarized in the thickness direction of the rectangular plate, and the secondary side is polarized in the longitudinal direction. When an AC voltage of a frequency in the vicinity of the resonance frequency of the piezoelectric transformer is applied to the primary electrodes, the piezoelectric transformer is caused to mechanically oscillate in the longitudinal direction. The mechanical oscillation is transformed into a voltage by the piezoelectric effect, so as to be taken out as a high-voltage power from the secondary electrodes.
FIG. 21 is a block diagram of a prior art driving circuit for the piezoelectric transformer shown in FIG. 20, i.e., a prior art cold cathode fluorescent lamp driving apparatus using a piezoelectric transformer. Conventional systems for driving a piezoelectric transformer include a self-excited oscillation circuit system and a separately excited oscillation circuit system. The self-excited oscillation circuit system has a problem in conversion efficiency, and contains drawbacks such as that the system cannot follow a large fluctuation of loads. Because of these reasons,in recent prior art examples, the separately excited oscillation circuit system is often used. The driving circuit shown in FIG. 21 is also a driving circuit of the separately excited system.
In FIG. 21, a variable oscillation circuit 101 generates an AC driving signal of a frequency in the vicinity of the resonance frequency of a piezoelectric transformer 104. The output signal of the variable oscillation circuit 101 is waveform-shaped into a substantially sinusoidal wave by a waveform shaping circuit 102 in order to reduce a loss in the piezoelectric transformer 104. As the waveform shaping circuit 102, a low pass filter is used in a simple case, and a bandpass filter is used in the case where the efficiency is significant. The output of the waveform shaping circuit 102 is subjected to current amplification or voltage amplification so as to have a level at which a driving circuit 103 can sufficiently drive the piezoelectric transformer. The driving circuit 103 is configured by only a usual amplifying circuit consisting of transistors, or by a combination of an amplifying circuit and a step-up transformer. The output of the driving circuit 103 is boosted by the piezoelectric transformer 104, and then applied to a cold cathode fluorescent lamp 105 so that the cold cathode fluorescent lamp 105 is lit.
The resonance frequency of the piezoelectric transformer 104 is varied because of changes in environments such as the temperature and the load. If a piezoelectric transformer is driven by a constant frequency as in the circuit shown in FIG. 21, therefore, the relationship between the piezoelectric transformer and the driving frequency is varied. When the driving frequency is largely deviated from the resonance frequency of the piezoelectric transformer, the voltage stepup ratio of the piezoelectric transformer is significantly reduced so that a sufficient current cannot be caused to flow through the cold cathode fluorescent lamp 105. Thus, the cold cathode fluorescent lamp 105 cannot keep sufficient brightness.
A circuit shown in FIG. 22 can comply with the variation in the resonance frequency of the piezoelectric transformer 104. FIG. 22 is a block diagram of another prior art driving circuit of the piezoelectric transformer 104 shown in FIG. 20, i.e., a prior art cold cathode fluorescent lamp driving apparatus using a piezoelectric transformer. Functions of a variable oscillation circuit 101, a waveform shaping circuit 102, a driving circuit 103, and a piezoelectric transformer 104 are the same as those in the circuit shown in FIG. 22. In the circuit shown in FIG. 22, a feedback resistor 106 having a small resistance is connected in series to the cold cathode fluorescent lamp 105 so that the current flowing through the cold cathode fluorescent lamp 105 is detected via the feedback resistor 106. The voltage across the feedback resistor 106 is input to an oscillation control circuit 107. The oscillation control circuit 107 controls the frequency of the output signal of the variable oscillation circuit 101 in such a manner that the voltage across the feedback resistor 106 is constant, i.e., the current flowing through the cold cathode fluorescent lamp 105 is constant. As a result of the control, the cold cathode fluorescent lamp 105 is lit with substantially constant brightness. At this time, the driving frequency is kept having a substantially constant relationship with the resonance frequency of the piezoelectric transformer.
In the above, the driving circuit of the separately excited oscillation circuit system has been described as a prior art example of the piezoelectric transformer driving apparatus.
If a cold cathode fluorescent lamp is driven by an AC voltage, however, the characteristics are greatly and drastically changed, that is, the absolute value and the phase of the impedance change greatly and drastically. In the case where the cold cathode fluorescent lamp is driven by an AC voltage of a high frequency, particularly, the changes are considerably large and complicated. In addition, if the tube diameter is reduced, the tendency greatly appears. In the above-described prior art piezoelectric transformer driving apparatuses, the above-mentioned changes of the cold cathode fluorescent lamp are not considered. Thus, the prior art driving apparatuses cannot comply with the changes, and the current flowing through the cold cathode fluorescent lamp is pulsated so that brightness cannot be kept constant. As a result, there exist problems in that the reliability of the cold cathode fluorescent lamp is reduced, and that the life period of the lamp is shortened.
If the current flowing through the cold cathode fluorescent lamp is pulsated, even the driving apparatus shown in FIG. 19 cannot control the current flowing through the cold cathode fluorescent lamp so as to be constant. Thus, the driving frequency cannot be kept having a substantially constant relationship with the resonance frequency of the piezoelectric transformer, so that the driving efficiency of the piezoelectric transformer is reduced and also the efficiency of the cold cathode fluorescent lamp driving apparatus using the piezoelectric transformer is reduced. In addition, the piezoelectric transformer is greatly disturbed by the pulsation so that heat generation is increased. As a result, there exists a problem in that the reliability is significantly degraded.